PROJECT SUMMARY/ABSTRACT Borderline personality disorder (BPD) is a severe mental health condition with high morbidity and mortality. Although dialectical behavior therapy (DBT) is an efficacious treatment for BPD, it is resource-intensive and lengthy in its full form, involving one year of weekly individual therapy and group skills training in mindfulness, emotion regulation, interpersonal effectiveness, and distress tolerance. As a result, few patients have access to the full treatment. A better understanding of how the distinct components of DBT affect different sets of symptoms could help to streamline this treatment and personalize its use with specific patients. DBT emotion regulation skills training may be a particularly important component of DBT. Emotional dysfunction is theorized to underlie many of the problems seen in BPD, including impairments in decision- making, impulsive behaviors, and self-injury. The PI's pilot data suggest that DBT emotion regulation skills may have unique effects on emotion targets in BPD, as well as broader effects beyond emotional symptoms. Conversely, DBT interpersonal effectiveness may affect interpersonal targets. Yet this small sample pilot remains the only study to test experimentally the effects of distinct DBT skills modules. Therefore, the proposed study examines the unique effects of two distinct DBT skills training modules. Participants will be adults with BPD and recent, recurrent self-injurious behaviors (N = 81) who will be randomly assigned to six weeks of DBT emotion regulation skills training (DBT-ER), DBT interpersonal effectiveness skills training (DBT-IE), or a non-skills control group. Using innovative laboratory-based assessment methods, the proposed study will examine the effects of these conditions on emotional responding and interpersonal functioning, as well as BPD related outcomes. Aim 1 examines the unique effects of DBT-ER and DBT-IE on their respective emotion-related (subjective and biological emotional reactivity, behavioral emotion regulation, skills use) and interpersonal (subjective and behavioral) targets, compared to the control. Aim 2 examines whether DBT-ER target engagement predicts reductions in BPD symptoms and self-injury. Aim 3 examines whether baseline high emotion dysregulation predicts greater response to the DBT-ER condition versus the other conditions. The research team (PI: Dixon-Gordon, Co-I: Gratz; Consultants: Cooney, Hazlett; Collaborators: Constantino, Laws) has extensive experience in DBT, emotion regulation, biological assessment of stress reactivity, and treatment research and is therefore uniquely positioned to address these aims. The proposed research is innovative in its experimental examination of the effects of DBT components on specific targets in BPD. Given the high societal costs of BPD, this work has important public health significance. Findings will inform an R01 submission examining modular use of DBT components to result in briefer and more efficient individualized treatments for patients.